The majority of generators, which are also known as generating sets or gensets, are operated at some point without synchronisation to the mains electricity supply and in many cases are used as an emergency back up supply to the mains electricity supply. Thus, when a load is connected to the generator, it should be able to accept a current inrush within certain parameters and these are regulated by industrial standards such as ISO 8528. The current inrush corresponds to a power surge commonly referred to as a load step, and the maximum load step that can safely be applied to an engine is generally regarded as being about half of the fuel stop power of the engine, i.e., the maximum power that the engine is capable of delivering transiently. In practice, therefore, when preparing a specification for a generator, the end user will require a generator with a power rating about twice the magnitude of the anticipated load step.
However, during continuous running of the generator, the running load on the generator will often be significantly lower than the load step and can be as little as 15-30% of the magnitude of the load step.
The load step requirements of the generator result in all the equipment and material associated with the generator, such as the engine room, silencers and canopies, in addition to the generator itself, being significantly oversized with respect to the requirements of the running load on the generator. An additional consequence of this is that the efficiency and performance of the generator is affected with common drawbacks being higher fuel consumption and CO2 emissions and excessive wear due to the low loads. This, in turn, leads to higher maintenance costs in addition to the higher capital costs of the oversize generator.
Different approaches to reducing this problem, such as load shedding, have been tried but the additional costs of these approaches are similar to the additional costs of an oversized generator. Therefore, these approaches have not been widely adopted. In practice, most commercial gensets cannot reach more than a load step of about 67% of the fuel stop power without a substantial risk of stalling. Such gensets would also fall outside the acceptance standards of the industry standard ISO 8528.
U.S. Pat. No. 4,862,058 discloses a DC power source having an artificial load that continuously regulates the voltage going to the actual load and focuses on maintaining a steady state on the load by creating a permanent voltage differential between the source and the load or tool in use. The approach suffers from a number of limitations that restrict its use to DC applications and it does not address any of the problems of the load step discussed above.
U.S. Pat. No. 6,239,511 discloses the use of an external load in conjunction with a turbine driven generator, which is connected to the mains electricity supply. Turbine driven generators are notoriously difficult to start and get up to synchronisation. As such, there is no load step requirement as the application of the load will be done by synchronising the generator to the mains. Load is then progressively and slowly applied to the generator. This approach is not relevant to the present invention, which deals with the problem where the load is applied without mains support, which generates a load step. The problem of the invention arises when there is a loss of mains power.